<?php
function div2p ($html) {
	$search = array('|<div[^>]*?>|is','/<\/div>/i', '/<p[^>]*?>/is');
	$replace = array('<p>','</p>','<p>');
	$html = preg_replace($search, $replace, $html);
	if ( false === strpos($html,'<p>') ) {
		$html = '<p>'.$html.'</p>';
	}
	return $html;
}

add_filter('post_excerpt', 'div2p');
?>